Conventionally, various different mobile telephones, sound output devices, or outgoing-talk/incoming-talk devices have been proposed for various different objectives. For example, to provide a mobile telephone permitting clear listening or outgoing-talking/incoming-talking even in the presence of loud noise, there has been proposed a mobile telephone in which a bone conduction speaker is employed, the mobile telephone being provided with the bone conduction speaker as well as with external auditory meatus stoppage means (Patent Document 1). On the other hand, in another proposed method for using a bone conduction speaker, a manual operation is used to adjust the pressure of contact between the tragus and a vibrating surface to be brought into contact with the tragus, whereby the ratio at which audio information through cartilage conduction and audio information through air conduction are transmitted can be altered in accordance with the magnitude of outside noise (Patent Document 2). In yet another proposal, a piezoelectric element is used as a vibration source of bone conduction. A further proposal for an outgoing-talk/incoming-talk device for a mobile telephone is a wireless communication function headset that is wirelessly communicatively connected to a communication apparatus capable of audio communication via a communication network, the wireless communication function headset permitting audio communication with a party on the line via the communication apparatus (Patent Document 3). In yet another proposal for an outgoing-talk/incoming-talk device, an eyeglasses-type interface device is provided with an audio unit that includes a bone conduction earphone, a microphone, and a display unit for displaying, on a lens, movie information that has been sent to a wireless communication unit from a mobile telephone or the like (Patent Document 4). It is typical for a “smartphone”-type mobile telephone to be sheathed in a soft cover, as an accessory device. Various other mobile telephone accessory devices, such as headsets for wired or short-range wireless communication with a mobile telephone, have also been proposed.